Now that I've had 2+, I'll never go back. Growing up, my family was always a 1-dog family, which was fine, but now that I've seen how much enjoyment the dogs get from each other, I don't think I'll be able to have just one. It's neat to have such diverse training options, different classes, different personalities. And truly, 3 dogs are less work than one for me. The dogs just automatically go to each other to play, making up their own games, wrestling, etc. So they wear each other out, which is fun. And when I play games with them, it's cool to see the fun competition between them (for instance, in stay/sit/down games, playing chuckit, etc).
The only downside with having three is triple the cost on classes and entry fees. When DH and I just had Honor, we didn't even think about how much we spent on her classes and stuff. But with three, it's definitely more of a budget-planning event. But it's worth it.
I think my top limit is going to be four. DH agrees. 

Lisa and:
URO1 'PR' AliAK's Vereesa Windrunner RL1, CGC, TDI - "Honor" - Alaskan Klee Kai
Victory At Marathon - "Nike" - Shetland Sheepdog
URO1 RBIS GRCH 'PR' AliAK's Ain't No Mountn Hi Enf, CGC, TDI - "Eli" - Alaskan Klee Kai
BIMBS BISS GRCH 'PR' Alaskai's Nikiski V AliAK, CGC, TDI - "Nikki" - Alaskan Klee Kai
Dubby - Holland Lop
Waiting At The Bridge: Max (Mini Lop Rabbit)